Support in tree hooks (pyproject.toml backend-path key)

pep517/_in_process.py

@@ -2,7 +2,9 @@
 
 It expects:
 - Command line args: hook_name, control_dir
-- Environment variable: PEP517_BUILD_BACKEND=entry.point:spec
+- Environment variables:
+      PEP517_BUILD_BACKEND=entry.point:spec
+      PEP517_BACKEND_PATH=paths (separated with os.pathsep)
 - control_dir/input.json:
   - {"kwargs": {...}}
 
@@ -13,6 +15,7 @@
 from glob import glob
 from importlib import import_module
 import os
+import os.path
 from os.path import join as pjoin
 import re
 import shutil
@@ -29,14 +32,41 @@ def __init__(self, traceback):
         self.traceback = traceback
 
 
+class BackendInvalid(Exception):
+    """Raised if the backend is invalid"""
+    def __init__(self, message):
+        self.message = message
+
+
+def contained_in(filename, directory):
+    """Test if a file is located within the given directory."""
+    filename = os.path.normcase(os.path.abspath(filename))
+    directory = os.path.normcase(os.path.abspath(directory))
+    return os.path.commonprefix([filename, directory]) == directory
+
+
 def _build_backend():
     """Find and load the build backend"""
+    # Add in-tree backend directories to the front of sys.path.
+    backend_path = os.environ.get('PEP517_BACKEND_PATH')
+    if backend_path:
+        extra_pathitems = backend_path.split(os.pathsep)
+        sys.path[:0] = extra_pathitems
+
     ep = os.environ['PEP517_BUILD_BACKEND']
     mod_path, _, obj_path = ep.partition(':')
     try:
         obj = import_module(mod_path)
     except ImportError:
         raise BackendUnavailable(traceback.format_exc())
+
+    if backend_path:
+        if not any(
+            contained_in(obj.__file__, path)
+            for path in extra_pathitems
+        ):
+            raise BackendInvalid("Backend was not loaded from backend-path")
+
     if obj_path:
         for path_part in obj_path.split('.'):
             obj = getattr(obj, path_part)
@@ -203,6 +233,9 @@ def main():
     except BackendUnavailable as e:
         json_out['no_backend'] = True
         json_out['traceback'] = e.traceback
+    except BackendInvalid as e:
+        json_out['backend_invalid'] = True
+        json_out['backend_error'] = e.message
     except GotUnsupportedOperation as e:
         json_out['unsupported'] = True
         json_out['traceback'] = e.traceback

pep517/wrappers.py

@@ -26,6 +26,14 @@ def __init__(self, traceback):
         self.traceback = traceback
 
 
+class BackendInvalid(Exception):
+    """Will be raised if the backend is invalid."""
+    def __init__(self, backend_name, backend_path, message):
+        self.backend_name = backend_name
+        self.backend_path = backend_path
+        self.message = message
+
+
 class UnsupportedOperation(Exception):
     """May be raised by build_sdist if the backend indicates that it can't."""
     def __init__(self, traceback):
@@ -41,15 +49,45 @@ def default_subprocess_runner(cmd, cwd=None, extra_environ=None):
     check_call(cmd, cwd=cwd, env=env)
 
 
+def norm_and_check(source_tree, requested):
+    """Normalise and check a backend path.
+
+    Ensure that the requested backend path is specified as a relative path,
+    and resolves to a location under the given source tree.
+
+    Return an absolute version of the requested path.
+    """
+    if os.path.isabs(requested):
+        raise ValueError("paths must be relative")
+
+    abs_source = os.path.abspath(source_tree)
+    abs_requested = os.path.normpath(os.path.join(abs_source, requested))
+    # We have to use commonprefix for Python 2.7 compatibility. So we
+    # normalise case to avoid problems because commonprefix is a character
+    # based comparison :-(
+    norm_source = os.path.normcase(abs_source)
+    norm_requested = os.path.normcase(abs_requested)
+    if os.path.commonprefix([norm_source, norm_requested]) != norm_source:
+        raise ValueError("paths must be inside source tree")
+
+    return abs_requested
+
+
 class Pep517HookCaller(object):
     """A wrapper around a source directory to be built with a PEP 517 backend.
 
     source_dir : The path to the source directory, containing pyproject.toml.
     backend : The build backend spec, as per PEP 517, from pyproject.toml.
+    backend_path : The backend path, as per PEP 517, from pyproject.toml.
     """
-    def __init__(self, source_dir, build_backend):
+    def __init__(self, source_dir, build_backend, backend_path=None):
         self.source_dir = abspath(source_dir)
         self.build_backend = build_backend
+        if backend_path:
+            backend_path = [
+                norm_and_check(self.source_dir, p) for p in backend_path
+            ]
+        self.backend_path = backend_path
         self._subprocess_runner = default_subprocess_runner
 
     # TODO: Is this over-engineered? Maybe frontends only need to
@@ -143,25 +181,40 @@ def _call_hook(self, hook_name, kwargs):
         # letters, digits and _, . and : characters, and will be used as a
         # Python identifier, so non-ASCII content is wrong on Python 2 in
         # any case).
+        # For backend_path, we use sys.getfilesystemencoding.
         if sys.version_info[0] == 2:
             build_backend = self.build_backend.encode('ASCII')
         else:
             build_backend = self.build_backend
+        extra_environ = {'PEP517_BUILD_BACKEND': build_backend}
+
+        if self.backend_path:
+            backend_path = os.pathsep.join(self.backend_path)
+            if sys.version_info[0] == 2:
+                backend_path = backend_path.encode(sys.getfilesystemencoding())
+            extra_environ['PEP517_BACKEND_PATH'] = backend_path
 
         with tempdir() as td:
-            compat.write_json({'kwargs': kwargs}, pjoin(td, 'input.json'),
+            hook_input = {'kwargs': kwargs}
+            compat.write_json(hook_input, pjoin(td, 'input.json'),
                               indent=2)
 
             # Run the hook in a subprocess
             self._subprocess_runner(
                 [sys.executable, _in_proc_script, hook_name, td],
                 cwd=self.source_dir,
-                extra_environ={'PEP517_BUILD_BACKEND': build_backend}
+                extra_environ=extra_environ
             )
 
             data = compat.read_json(pjoin(td, 'output.json'))
             if data.get('unsupported'):
                 raise UnsupportedOperation(data.get('traceback', ''))
             if data.get('no_backend'):
                 raise BackendUnavailable(data.get('traceback', ''))
+            if data.get('backend_invalid'):
+                raise BackendInvalid(
+                    backend_name=self.build_backend,
+                    backend_path=self.backend_path,
+                    message=data.get('backend_error', '')
+                )
             return data['return_val']

tests/samples/pkg_intree/backend/intree_backend.py

@@ -0,0 +1,2 @@
+def get_requires_for_build_sdist(config_settings):
+    return ["intree_backend_called"]

tests/samples/pkg_intree/pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+build-backend = 'intree_backend'
+backend-path = ['backend']

tests/test_inplace_hooks.py

@@ -0,0 +1,49 @@
+from os.path import dirname, abspath, join as pjoin
+import pytoml
+from testpath import modified_env
+import pytest
+
+from pep517.wrappers import Pep517HookCaller, BackendInvalid
+
+SAMPLES_DIR = pjoin(dirname(abspath(__file__)), 'samples')
+BUILDSYS_PKGS = pjoin(SAMPLES_DIR, 'buildsys_pkgs')
+
+
+def get_hooks(pkg, backend=None, path=None):
+    source_dir = pjoin(SAMPLES_DIR, pkg)
+    with open(pjoin(source_dir, 'pyproject.toml')) as f:
+        data = pytoml.load(f)
+    if backend is None:
+        backend = data['build-system']['build-backend']
+    if path is None:
+        path = data['build-system']['backend-path']
+    return Pep517HookCaller(source_dir, backend, path)
+
+
+def test_backend_path_within_tree():
+    source_dir = pjoin(SAMPLES_DIR, 'pkg1')
+    assert Pep517HookCaller(source_dir, 'dummy', ['.', 'subdir'])
+    assert Pep517HookCaller(source_dir, 'dummy', ['../pkg1', 'subdir/..'])
+    # TODO: Do we want to insist on ValueError, or invent another exception?
+    with pytest.raises(Exception):
+        assert Pep517HookCaller(source_dir, 'dummy', [source_dir])
+    with pytest.raises(Exception):
+        Pep517HookCaller(source_dir, 'dummy', ['.', '..'])
+    with pytest.raises(Exception):
+        Pep517HookCaller(source_dir, 'dummy', ['subdir/../..'])
+    with pytest.raises(Exception):
+        Pep517HookCaller(source_dir, 'dummy', ['/'])
+
+
+def test_intree_backend():
+    hooks = get_hooks('pkg_intree')
+    with modified_env({'PYTHONPATH': BUILDSYS_PKGS}):
+        res = hooks.get_requires_for_build_sdist({})
+    assert res == ["intree_backend_called"]
+
+
+def test_intree_backend_not_in_path():
+    hooks = get_hooks('pkg_intree', backend='buildsys')
+    with modified_env({'PYTHONPATH': BUILDSYS_PKGS}):
+        with pytest.raises(BackendInvalid):
+            hooks.get_requires_for_build_sdist({})